1. Field of the Invention
The invention relates to prealloyed high-vanadium, cold work tool steel particles for use in the powder-metallurgy production of cold work tool steel articles and to a method for producing these particles.
2. Description of the Prior Art
In various high-vanadium cold work tool steel applications, high wear resistance in combination with good grindability, strength and toughness are required. U.S. Pat. No. 4,249,945 discloses tool steel articles made by powder- metallurgy techniques using alloys such as AISI A-11. These articles are made in the conventional manner from compacted, prealloyed particles that contain relatively large volumes of MC-type vanadium carbides to provide improved wear resistance. These articles exhibit a good combination of wear resistance, toughness and strength; however, for some applications the wear resistance is not adequate.
In alloys of this type, it is known that the wear resistance may be increased by increasing the MC-type vanadium carbide content. MC-type vanadium carbide is particularly useful for this purpose because its hardness (microhardness of 2800 Kg/mm.sup.2) is greater than that of most other metallic carbides such as columbium carbide (microhardness of 2400 Kg/mm.sup.2), tantalum carbide (microhardness of 1800 Kg/mm.sup.2) and chromium carbide (microhardness of 1300 Kg/mm.sup.2). Increases in vanadium carbide content, however, typically result in degradation with respect to toughness. Specifically, it is generally accepted that vanadium contents of over 11% by weight result in degradation of toughness to levels unacceptable for many tool steel applications. Specifically in this regard, with vanadium contents in excess of 11%, the resulting size and dispersion of the MC-type vanadium carbides in the microstructure of the alloy detrimentally affects grindability, as well as toughness, of the alloy. Grindability is an important property of these alloys, because grinding is a necessary operation in producing final products, such as work rolls, punches, dies, plastic molds, slitter knives, plastic extrusion barrels, pump components and the like.